1. Technical Field
The invention relates to saturated power amplifiers in which the output power of the power amplifier is stabilized against environmental and device-to-device variations through a feedback control circuit.
2. Related Art
In today""s society, both the presence and use of communication systems are increasing at a rapid pace. Wireless and broadband communication systems and infrastructures continue to grow. This acceleration has created a strong and ever-growing market for electronic equipment that employs more powerful, efficient, and inexpensive communication components and devices.
Electronic equipment such as computers, wireless devices, broadband devices may communicate with one another by passing transmission signals through free-space space (i.e., air and space) and through guided mediums such as wire, cable, microwave, millimeter wave, sonic, and optical connections. These transmission signals may experience a variety of processing steps during their communication including the process of amplifying the transmission signals.
As an example, in order to amplify the transmission signals in a mobile telephone (also typically known as a xe2x80x9ccellular telephone,xe2x80x9d xe2x80x9ccellular phonexe2x80x9d or xe2x80x9ccellphonexe2x80x9d), a cellphone manufacturer may purchase, from another vendor, an integrated circuit chip that includes an amplifier (such as a power amplifier). Typically this power amplifier will vary the power of a transmission signal from a power level at an input of the power amplifier to a new desired power level at an output of the power amplifier, in response to a received power control signal (such as a power control voltage) from a supply source in the cellphone. Unfortunately, power amplifiers have certain characteristics (including gain, linearity, saturation point, peak-power, efficiency and other electrical properties that include all the parameters of the transistors in the power amplifier) that vary from power amplifier to power amplifier (i.e., device-to-device).
These characteristics are typically susceptible to environmental variations such as changes in temperature, power supply voltage and process changes (such as the variations caused in the manufacturing process) and vary from device-to-device. In order to compensate for these variations in characteristics, at present, cellphone manufacturers typically test each power amplifier individually and then generate software algorithms and/or custom calibration tables that compensate for the variations.
To this end, cellphone manufacturers will generally utilize a control chip (that is separate from the power amplifier and may include the software algorithms and/or custom calibration tables) that performs a calibration process on the power amplifier. Additionally, the vendors spend time calibrating and testing each power amplifier to ensure that each power amplifier complies with system specifications provided by the cellphone manufacturer.
Moreover, on purchase and installation of a vendor supplied power amplifier, the cellphone manufacturer must spend time calibrating the working relationship between the control chip and the power amplifier to ensure that the relationship complies with system specifications. As a result of this manufacturer calibration process, the power amplifier output power may vary consistently and within specifications with an applied power control voltage profile.
A problem with the conventional testing process performed by the vendor and the calibration processes performed by the vendor and the manufacturer is that they generally are labor intensive, subject to many errors, and generally increase the cost of the manufactured products. It is desirable to minimize the problems associated with incorporating a vendor supplied power amplifier into a communication device.
A system is disclosed that works through local feedback, enabling a power amplifier with a control voltage output power characteristic that is invariant to environ mental and part-to-part process variations. Broadly conceptualized, the system may include a power amplifier coupled to a local controller. A power control voltage is applied to the local controller along with a feedback signal proportional to the power amplifier output power. The local controller compares these two signals. This results in an error voltage that is applied to the power amplifier control voltage input. Through the feedback loop action, the error voltage is minimized causing the amplifier output power to follow the applied power control voltage without sensitivity to environmental (i.e., temperature, input power, or supply voltage) or power amplifier variations.
The signal that is fed back into the local controller may be either a direct or indirect representation of the power amplifier output power. For example, a direct representation may be the direct current (DC) voltage resulting from a power detector fed by a directional coupler at the power amplifier output. Under closed loop control, the output power control characteristic would be proportional to the input control voltage. Indirect representations of the output power may be the DC supply voltage, base, or collector currents of the power amplifier. Another indirect representation may be the detected RF voltage at the power amplifier output or at any voltage node along the output impedance match. Since these indirect signals are proportional to the square-root of the power, the closed loop output power control characteristic is non-linear with respect to the input control voltage.
Passing either a direct or indirect power feedback signal through a logarithmic amplifier before applying it to the error amplifier results in an output power control characteristic that has a linear-in-decibel (linear-in-dB) relationship with respect to the input control voltage. The same linear-in-dB characteristic can also be achieved by passing the input control voltage through an exponential amplifier before application to the error amplifier.
At high power outputs, components of the above system such as the power amplifier may reach a saturation point and cease to work properly. To extend the closed loop operational range of the power amplifier for example, a saturation detection feedback loop may be incorporated into the system to alter the power control voltage applied to the power amplifier.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.